1. Field of the Invention
The present invention relates to lithographic apparatus and, more particularly, to correcting wafer substrate deformations in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion or target field of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask (i.e., reticle), may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target field (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions or fields that are successively exposed. Known lithographic apparatus include so-called “steppers,” in which each target field is irradiated by exposing an entire pattern onto the target field using stationary illumination, and so-called “scanners,” in which each target field is irradiated by scanning the pattern through the projection beam in a given direction (e.g., the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion/field”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool.
Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target field of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target field of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target field, such as an integrated circuit.
Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
A support structure supports (i.e. bears the weight), of the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.”
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g. water), so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
The demand for smaller and smaller semiconductor devices drives the need to have lithographic fabrication processes achieve pattern features and profiles having smaller critical dimensions (“CDs”). Moreover, as indicated above, such devices may comprise multiple layers, requiring precise positioning of successive layers over a prior layers. Needless to say, it is important that these smaller devices are consistently reproduced with as little overlay errors as possible to yield high-quality wafer substrates W.
There are, however, numerous activities during the lithographic fabrication process that contribute to overlay errors and compromise the quality of the exposed patterns. Indeed, the very lithographic exposure process used to project the pattern onto the individual target fields C may contribute to overlay errors. In particular, the energy applied to a target field C during exposure is absorbed by the wafer substrate W as thermal energy. The absorbed thermal energy may cause the target field C under exposure to deform. Such deformations may result in unacceptable overlay and focusing errors and significantly reduce yield production.
Typically, in an effort to reduce pattern shifts and wafer substrate deformations, lithographic fabrication schemes employ a correction offset procedure to determine, and compensate for, a host of processing-related errors. These procedures are based on determining parameters for an average target field Cave and then determining corrective offsets for the average target field Cave. The corrective offsets are then fed back to a process control module that implements the offsets to recalibrate the various processes in order to comply with the characteristics of the average target field Cave. However, given the variety of thermally-induced deformations that can occur during exposure, correcting such deformations based on an average target field Cave proves to be less than optimum—if not inadequate—for many cases.